1. Field of the Invention
The present invention relates to apparatus and methods for improving the quality of the acceleration data output of a closed-loop accelerometer. More particularly, this invention pertains to a method and apparatus for reducing moding in the output of a digitally rebalanced accelerometer.
2. Description of the Prior Art
Many accelerometers include a pendulous proofmass for sensing acceleration relative to inertial space. Motion of the proofmass relative to the body of the accelerometer (and the attached object) is induced by acceleration and the displacement of the proofmass provides a measure of the acceleration force.
The displacement-versus-acceleration characteristic of an accelerometer is highly nonlinear. Such nonlinearity may be due to a number of factors. For example, in a silicon accelerometer that employs a hinged floating element between a pair of conductive plates, an inverse square relationship governs the force exerted upon the pendulous mass as it travels through the gap between the charged conductive plates. Nonlinearities may also be introduced through the bending characteristics of the hinges that attach the pendulous mass to the frame. Such second order effects can produce a nonlinear stiffness response in the bent hinge as the pendulous mass is deflected through the gap.
In order to improve the accuracy of such accelerometers, they are often configured to operate in a closed-loop fashion. The force then required to maintain the null position of the pendulous mass, rather than its displacement, provides the measure of acceleration. A rebalance loop is employed for this purpose and many rebalance techniques are available. Generally, the most effective techniques employ a modulation and forcing process that takes place at a specific frequency. The output of the accelerometer may be represented by a digital value that is equal to the duty cycle of the voltage rebalance waveform in voltage rebalance schemes. Alternatively, the digital value may represent the number of charge quanta applied in charge rebalance schemes. Closed loop accelerometers of the above-identified type are illustrated, for example, in U.S. Pat. No. 4,679,434 entitled xe2x80x9cIntegrated Force Balanced Accelerometerxe2x80x9d and U.S. Pat. No. 5,142,921 entitled xe2x80x9cForce Balance Instrument With Electrostatic Charge Controlxe2x80x9d of Robert E. Stewart et al. They include a pendulous mass and a pair of electrostatic plates or electrodes positioned close to, but slightly spaced from and on opposite sides of the mass that act as both pickoffs and forcers. The mass is cantilevered from a surrounding frame by hinges to deflect about an output axis in response to acceleration along a predetermined input axis.
Force rebalance control of the position of the mass is achieved by controlling the charge on the capacitor plates. The devices are operated by repetitively applying a constant attractive force that acts alternately on opposed sides of the sensing mass with the fixed force applied to one or the other side for varying intervals. The relative lengths of successive intervals are determined by the magnitude of the acceleration detected by the hinged mass.
The relative lengths of the successive intervals or part cycles of force application are controlled by varying the duty cycle of a periodic wave (e.g. a square wave). For example, when the duty cycle is fifty percent, equal and opposite forces are applied to the pendulum for equal periods of time and the resultant force on the sensing mass is zero. The difference in duration between the two portions of a single cycle, ITORQ, is a linear measure of acceleration.
In a rebalanced system such as that described above, it is often advantageous to convert the measure of acceleration (i.e. the differential duration of time for returning the mass 10 to null, ITORQ) to digital form prior to application to the electrodes acting as torquers.
The digital value could be, for example, the number of charge quanta applied to the electrodes. Since the force required to null the proofmass provides accelerometer output, the value of acceleration comprises a stream of pulses with the average amplitude over a number of force rebalance cycles being the measure of acceleration.
Should the acceleration over a given number of force rebalance cycles include a fractional value, this will be expressed, in a digitized format, by sequences of digital words that comprise integer values which, averaged together, express the correct fractional amount. It has been observed that the resultant sequences of digitized values can contain low-frequency, non-random patterns known as xe2x80x9cmoding noisexe2x80x9d. The often arbitrarily-low frequencies of such patterns within output data complicate its interpretation, analysis and reliability.
The preceding and other problems of the prior art are addressed by the present invention that provides, in a first aspect, a method for reducing periodic low frequency noise in the output of a closed-loop force rebalanced accelerometer of the type in which a quantized digital signal is used as a nulling signal for torquing a pendulous mass. The digital signal is also used to provide a measure of an input quantity such as acceleration. Such method is begun by receiving the digitized output. Thereafter, the digitized output signal is processed using methods that will alternate low frequency patterns resulting from the quantization while preserving true low frequency information.
In a second aspect, the invention provides a method for reducing periodic low frequency noise in the output of a closed-loop force rebalanced accelerometer of the type in which a digital output signal for torquing a pendulous mass is derived from an analog nulling signal. Such method is begun by adding analog noise to the nulling signal. Thereafter, the resultant signal is digitized to form the output signal.
In a third aspect, the invention provides a method for reducing periodic low frequency noise in the output of a closed-loop force rebalanced accelerometer of the type in which an analog nulling signal is digitized to form a signal that is fed back within the closed loop. Such method comprises the step of randomizing the digitized signal to form the output.
In a fourth aspect, the invention provides a closed-loop force rebalanced accelerometer. Such accelerometer includes a pendulous mass. A pickoff circuit generates an analog signal responsive to displacement of the mass. A transfer function circuit generates an analog nulling signal. An analog-to-digital conversion circuit is provided for digitizing the nulling signal. A torquer circuit is provided for driving the pendulous mass in response to the digitized nulling signal. A filter, located outside the closed loop, receives the digitized nulling signal and generates an output in response.
In a fifth aspect, the invention provides a closed-loop force rebalanced accelerometer that includes a pendulous mass, a pickoff circuit for generating an analog signal in response to displacement of the mass, a transfer function circuit for generating an analog nulling signal in response to the pickoff signal, an analog-to-digital conversion circuit for digitizing the nulling signal and a torquer circuit for driving the pendulous mass in response to the digitized nulling signal. A circuit is additionally provided within the loop for adding analog noise to the nulling signal whereby the nulling signal including analog noise is digitized by the analog-to-digital conversion circuit.
Finally, in a sixth aspect, the invention provides a closed-loop force rebalanced accelerometer generally as above. However, rather then providing apparatus for adding analog noise, in this aspect a circuit is provided for randomizing the nulling signal.
The foregoing and other features and advantages of this invention will become apparent from the detailed description that follows. Such detailed description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the drawing figures and the written text.